Читать книгу Modern Trends in Structural and Solid Mechanics 3 - Группа авторов - Страница 18

Mitochondrial motility, or trafficking, is critical for the survival of all cells. Neurons, which extend their axons and dendrites up to a meter, between two and three orders of magnitude greater than most other cells, are especially vulnerable to any inability by their mitochondria to get to sites with a high energy demand. At any instant of time, about 10–40% of the mitochondria are generally moving, with about half of those moving away from the cell body (anterograde, kinesin-dependent) and the rest towards the cell body (retrograde, dynein-dependent) (Schwarz 2013).

The distribution of mitochondria over long distances in the neurons is regulated by a complex molecular machinery that has evolved to match the very dynamic demand for energy with an optimal mitochondrial distribution (Schwarz 2013). Highly branched paths of the complex neuron geometry must be navigated, knowing where and when to stop. Machinery for fission and fusion can intersect with machinery for motility using feedforward and feedback mechanisms. Misregulation of motility can lead to neurodegeneration (Vanhauwaert et al. 2019).

The dendritic synapses are where the neurons receive signals from other neurons. Energy demand is greatest at the synapses and can change rapidly in response to almost instantaneous environmental changes. The same is true at the axonal synapses, which are used by neurons to transfer signaling downstream to subsequent neurons. Neuronal axons lie flat and are typically about a micron in diameter. Trafficking is along linear arrays of uniformly polarized microtubules, where the negative ends of the tubules are anchored in the cell body and the positive charge ends in the distal tips. Due to the local morphology, axonal mitochondria have separated from the reticulum and exist as discrete organelles of a dimension, typically 1–3 microns, with those in dendrites tending to be longer.

Mitochondria are a fundamental component for healthy living, supporting optimal functioning and efficient energy usage at all levels, thus avoiding numerous pathologies. In the discussion so far, we maintain that a significant controlling aspect of mitochondrial functioning is based on optimizations of a variety of defining characteristics. Here, motility and the placement of mitochondria within dendrites and axons can be viewed as optimal solutions, assuring sufficient energy at locations of high energy demand.